As several Republican presidential candidates embrace a flat tax, one contender stands out in not doing so: Mitt Romney, who has a long record of criticizing such plans and famously derided Steve Forbes’s 1996 proposal as a “tax cut for fat cats.”

But wait. Does Mitt Romney still think the plan is flawed? “I love a flat tax,” he said in August.

Mr. Romney’s support of the concept of a flat tax underscores the tightrope he is walking as taxes become a larger focus of the Republican presidential race and he faces rivals’ accusations of inconsistency on the issues.

Mr. Romney also is always careful to emphasize that he would never support any plan that hurts the middle class and helps the wealthy. But by replacing the graduated income tax with one single rate everyone pays, that is precisely what flat tax plans generally do, at least those that try to generate anywhere near the same tax revenue.

Mr. Romney’s favorable comments about flat taxes speak to the deep frustration many Republican voters share about the current system. While Herman Cain’s “9-9-9” proposal has been criticized because it includes a new national sales tax as well as a flat income tax, the catchy plan has helped vault the relatively unknown business person to the fore of the party’s field. Gov. Rick Perry of Texas will unveil a flat-tax proposal resembling the one put forward 15 years ago by Mr. Forbes, who is advising him.

Romney aides dispute the criticism and say his objection to the Forbes plan was specific: it would raise taxes on the middle class. A Romney spokeswoman said there was “no inconsistency” in his position. She said he could support a flat tax that did not raise taxes.

When asked about the flat-tax plans that have been floated in the last two decades, Romney aides could not recall any that might pass muster with Mr. Romney. Nor would they venture the outlines of a new plan that might meet his test. They also do not dispute the notion that a flat tax could never generate the same amount of tax revenue while also maintaining the same relative burdens on the wealthy and the middle class.

Analysts say it is hard to imagine a flat-tax plan that would not be very regressive. Now, the highest tax rate for many middle-income earners is 15%, while the richest Americans are subject to a 35% rate on much of their income.